Scientists have found that long-term exposure to urban air pollution known as ozone increases the risk of death from lung disease, and that the risk is greatest in large metropolitan areas.

According to a large U.S. study, researchers conclude that the risk of dying from lung disease associated with ozone exposure is three times higher in large cities compared to smaller urban areas, where pollution concentrations are lower.

Ozone is a form of air pollution, commonly known as smog, which is produced by the interaction of tailpipe and factory emissions and sunlight.

Michael Jerrett of the University of California at Berkeley led the study and says short term studies have been done suggesting a link between ozone and lung diseases but the latest study is different both in size and scope.

"What we found was that for the very first time, we were able to determine an association between ozone levels and premature mortality from respiratory causes," Jerrett said.

Researchers looked at the health impacts of ground level ozone on almost 450,000 people living in 96 large and small cities in the United States between 1977 and 2000. They found approximately 10,000 people died of some form of respiratory illness, with the greatest number of deaths occurring in large urban areas where smog levels are highest.

Jerrett says ozone is a lung irritant that causes asthma, chronic obstructive pulmonary disease and pneumonia which over time can lead to death. He says researchers found that ozone contributes to lung disease regardless of age, gender or smoking status.

"So, the important take home message there is that ozone is a pollutant that affects everyone and we need to be particularly concerned about those types of pollutants that are going to affect broad populations because the public health impact can be very large," he said.

The study on the health impact of ozone was published this week in the New England Journal of Medicine.